FERROELASTIC PHASE TRANSITION IN CaTiO₃ PEROVSKITE Simon A T Redfern Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom.

High-temperature (293 - 1523 K) X-ray powder diffraction has been used to investigate the nature of the transition from orthorhombic Pbnm to higher-symmetry polymorphs in CaTiO₃. While the behavior of perovskite is often quated as a textbook example of a displacive phase transition, the nature of the high-symmetry phase in the type-mineral, CaTiO₃, has been the subject of recent debate [1,2,3]. There have been suggestions that the structure transforms to an intermadiate tetragonal phase prior to the transition to cubic symmetry. In this investigation, the first X-ray powder diffraction study through the ferroelastic transition, it is demonstrated that CaTiO₃ transforms directly from Pbnm to Pm3m on increasing temperature at around 1385 K. The behavior is similar to that observed in neighborite, NaMgF₃, at high-temperature[4]. The temperature-dependence of the spontaneous strain in the ferroic phase has been determined, and shows that while the transition is weakly first-order, the strain conforms to near-second-order behavior over a wide temperature interval. These results provide no evidence for the existence of an intermediate tetragonal phase, or of cell-doubling within the orthorhombic stability field, nor of a higher temperature transition near 1520 K as was suggested by previous calorimetric[1] and inferred from lower-temperature X-ray studies[2].

References:

1. F Guyot, P Richet, Ph Courtial, Ph Gillet (1993) Phys Chem Minerals,

20, 141-146

2. X Liu, R C Liebermann (1993) Phys Chem Minerals, 20, 171-75

3. T Vogt, W W Schmahl (1993) Europhys Lett, 24, 281-285.

4. Y Zhao, D J Weidner, J B Parise, D E Cox (1993) Phys Earth Planet Int, 76, 1-16.